In recent years, there has been a trend towards more rapid processing of photographic material, such as silver halide films, together with reduced replenishment rates. Typically, graphic arts processors have identical path lengths in developer, fixer and wash baths. This enables identical racks to be used with a consequent reduction in cost of the processing machine. With equal development and fixing times, therefore, the rate determining process step has typically been development. A user sets a development time, which is sufficiently long to produce good quality results and the fixing time is automatically set to the same figure. Since the time required for adequate fixing has generally been shorter than the time required for adequate development, this has not been a problem. However, under certain conditions, where development times are shortened, or where for some reason, the time required for adequate fixing is increased, then fixing may become the rate determining process step.
Similar situations can also arise in colour processes, where the development and fixing times may not be equal, but where they are linked in a fixed ratio.
It can occur that the by-products of a particular processing step may actually retard the rate of reaction of that processing step. For example, the halide ions released by the development of silver halide crystals can retard the development reaction. One of the key components in a seasoned fixing stage which retards fixing rate is silver. It is known from, for example U.S. Pat. No. 5,736,304, that fixing rate shows an inverse dependence on fixing stage silver concentration. In the case of certain graphic arts recording films, such as Kodak GEN5 Film GRD™ for example, the fixing time is increased by approximately 1.2 seconds per g/l increase in the silver concentration at a constant fixer replenishment rate. Thus, 6 seconds extra are required to fix a film when the silver concentration in the fixing stage is 10 g/l than when the concentration is 5 g/l.
For a given fixing time each type of photographic material has an upper limit for fixing stage silver concentration at which point the material is just adequately fixed. It is clearly desirable to operate a photographic process in such a way that the silver concentration is less than the upper limit. This may be achieved either by setting the process conditions to ensure a high value of the upper limit or by maintaining a low value of silver concentration.
To address this, methods have been proposed, which rely on using an elevated fixer solution temperature to ensure a high value of the upper limit. However, there are a number of disadvantages to this approach. These include increased vapours given off from the fixing stage and an increased tendency for crystals to form on rollers and tank walls of the fixing stage. In addition, energy consumption is increased and there is a greater tendency for the fixer solution to “creep” up the tank wall with the consequent increased risk of contaminating adjacent tanks.
A method of keeping the silver concentration below the upper limit involves setting the fixer solution replenishment rate so that on average, after a suitably large area of film has been processed, the silver concentration will equilibrate to a value less than the upper limit. If the user makes a substantial change to the average image density on the photographic material then it may be necessary to change the fixer solution replenishment rate, but normally this approach works well, although it may require higher fixer solution replenishment rates than are desirable from a cost or environmental viewpoint.
A problem arises, however, when a user requires rapid processing as well as low fixer solution replenishment rates. The rapid processing requirement decreases the upper limit whereas the low fixer solution replenishment rate increases the silver concentration.
In these circumstances one approach is to reduce the silver concentration by recirculating the fixer solution through a silver recovery unit, which removes silver and then returns the de-silvered solution to the fixing stage. In-line electrolytic silver recovery is a well known example of this technique. Examples of other possible approaches use chemical precipitation, ion exchange or metal exchange for the treatment technology.
Electrolytic silver recovery has a further benefit of reducing fixer replenishment rates as well, since silver complexes are split during electrolysis, with the silver plating out on the cathode of the silver recovery unit and fixing agent being regenerated. Reductions in replenishment rate of up to a factor of two are possible when using in-line electrolytic silver recovery. Furthermore, the lower average silver concentration in the fixing stage results in less silver salts being carried over into downstream processing baths. For black and white processors, this results in a lower silver concentration in the wash bath and for many countries, this enables users to discharge wash water directly to drain without exceeding discharge limits.
A problem with most electrolytic silver recovery units is that they are not able to remove the silver at the same rate as it is introduced to the fixer during film processing. Indeed, for a current of 1 Amp, which is typical of many small silver recovery units, the ratio of the rate of input of silver to the rate of removal of silver by electrolysis is approximately 40 to 1. Clearly, these units are not able to cope with high throughput peaks and maintain the silver concentration below the upper limit.
The problem of reaching the upper limit is more likely to occur with users who batch their film processing. This may happen for example if a roll of film is exposed off-line and then the whole roll is processed in one go. Another situation which might arise is when jobs are sent to a batch queue and then the whole batch is run together—possibly overnight. The batch queue might also be used for an on-line exposing system where several users are sending jobs to a batch queue. If jobs arrive at the queue faster than they can be exposed and processed, the queue will lengthen and the processing will be continuous.
United Kingdom Patent No. GB 2,004,310 in the name of L'Accessorio Radiografico SpA discloses the use of in-line electrolytic silver recovery to maintain the silver concentration of the fixer solution below a predetermined level. European Patent Application having publication number EP 0,279,479 and U.S. Pat. No. 4,744,874 in the name of Toulson describe a two-stage electrolytic treatment of fixer solution. The first stage is an in-line unit in which the fixer solution is subjected to electrolysis using a current of between 0.3 and 2 Amps and the second stage is a terminal treatment unit prior to the fixer solution being discarded. The fixer solution is continuously recirculated from the fixing stage through the in-line unit and back to the fixing stage.
Problem to Be Solved By the Invention
A system for and method of processing photographic material is required that can provide rapid processing at low fixer solution replenishment rate. In particular, for processing systems that are equipped with in-line fixer solution treatment systems, a method is required which can ensure that photographic material processed is always adequately fixed, even under conditions of high peak throughput, without wasting fixer solution and without requiring continuous high fixer solution temperature.